The present invention relates generally to nip presses used to exert pressing forces on moving webs for the formation of, for example, paper, textile material, plastic foil and other related materials. In particular, the present invention is directed to methods and apparatus for measuring and removing the effects of rotational variability from the nip pressure profile of nip presses which utilize imbedded sensors in covered rolls. While prior art presses which utilize rolls with imbedded sensors may be capable of detecting variations in pressure along the length of the roll, these same imbedded sensors may not be capable of measuring and compensating for rotational variability that can be generated by the high speed rotation of the covered roll. The present invention provides a method and apparatus for measuring and removing rotational variability from the nip pressure profile of the covered roll so as to obtain a more true profile of the nip pressure being developed in the nip region.
Nipped rolls are used in a vast number of continuous process industries including, for example, papermaking, steel making, plastics calendering and printing. The characteristics of nipped rolls are particularly important in papermaking. In the process of papermaking, many stages are required to transform headbox stock into paper. The initial stage is the deposition of the headbox stock, commonly referred to as “white water,” onto a paper machine forming fabric, commonly referred to as a “wire.” Upon deposition, the a portion of the white water flows through the interstices of the forming fabric wire leaving a mixture of liquid and fiber thereon. This mixture, referred to in the industry as a “web,” can be treated by equipment which further reduce the amount of moisture content of the finished product. The fabric wire continuously supports the fibrous web and advances it through the various dewatering equipment that effectively removes the desired amount of liquid from the web.
One of the stages of dewatering is effected by passing the web through a pair or more of rotating rolls which form a nip press or series thereof, during which liquid is expelled from the web via the pressure being applied by the rotating rolls. The rolls, in exerting force on the web and fabric wire, will cause some liquid to be pressed from the fibrous web. The web can then be advanced to other presses or dry equipment which further reduce the amount of moisture in the web. The “nip region” is the contact region between two adjacent rolls through which the paper web passes. One roll of the nip press is typically a hard steel roll while the other is constructed from a metallic shell covered by a polymeric cover. However, in some applications both roll may be covered. The amount of liquid to be pressed out of the web is dependent on the amount of pressure being placed on the web as it passes through the nip region. Later rolls in the process at the machine calender are used to control the caliper and other characteristics of the sheet. Covered rolls are at times used at the calender. The characteristics of the rolls are particularly important in papermaking as the amount of pressure applied to the web during the nip press stage can be critical in achieving uniform sheet characteristics.
One common problem associated with such rolls can be the lack of uniformity in the pressure being distributed along the working length of the roll. The pressure that is exerted by the rolls of the nip press is often referred to as the “nip pressure.” The amount of nip pressure applied to the web and the size of the nip can be important in achieving uniform sheet characteristics. Even nip pressure along the roll is important in papermaking and contributes to moisture content, caliper, sheet strength and surface appearance. For example, a lack of uniformity in the nip pressure can often result in paper of poor quality. Excessive nip pressure can cause crushing or displacement of fibers as well as holes in the resulting paper product. Improvements to nip loading can lead to higher productivity through higher machine speeds and lower breakdowns (unplanned downtime).
Conventional rolls for use in a press section may be formed of one or more layers of material. Roll deflection, commonly due to sag or nip loading, can be a source of uneven pressure and/or nip width distribution. Worn roll covers may also introduce pressure variations. Rolls have been developed which monitor and compensate for these deflections. These rolls generally have a floating shell which surrounds a stationary core. Underneath the floating shell are movable surfaces which can be actuated to compensate for uneven nip pressure distribution.
Previously known techniques for determining the presence of such discrepancies in the nip pressure required the operator to stop the roll and place a long piece of carbon paper or pressure sensitive film in the nip. This procedure is known as taking a “nip impression.” Later techniques for nip impressions involve using mylar with sensing elements to electronically record the pressures across the nip. These procedures, although useful, cannot be used while the nip press is in operation. Moreover, temperature, roll speed and other related changes which would affect the uniformity of nip pressure cannot be taken into account.
Accordingly, nip presses were developed over the years to permit the operator to measure the nip pressure while the rolls were being rotated. One such nip press is described in U.S. Pat. No. 4,509,237. This nip press utilizes a roll that has position sensors to determine an uneven disposition of the roll shell. The signals from the sensors activate support or pressure elements underneath the roll shell, to equalize any uneven positioning that may exist due to pressure variations. The pressure elements comprise conventional hydrostatic support bearings which are supplied by a pressurized oil infeed line. The roll described in U.S. Pat. No. 4,898,012 similarly attempts to address this problem by incorporating sensors on the roll to determine the nip pressure profile of a press nip. Yet another nip press is disclosed in U.S. Pat. No. 4,729,153. This controlled deflection roll further has sensors for regulating roll surface temperature in a narrow band across the roll face. Other controlled deflection rolls such as the one described in U.S. Pat. No. 4,233,011, rely on the thermal expansion properties of the roll material, to achieve proper roll flexure.
Further advancements in nip press technology included the development of wireless sensors which are imbedded in the sensing roll covers of nip presses as is disclosed in U.S. Pat. Nos. 7,225,688; 7,305,894; 7,392,715; 7,581,456 and 7,963,180 to Moore et al. These patents show the use of numerous sensors imbedded in the roll cover, commonly referred to as a “sensing roll,” which send wireless pressure signals to a remote signal receiver. U.S. Pat. No. 5,699,729 to Moschel discloses the use of a helical sensor for sensing pressure exhibited on a roll. Paper machine equipment manufacturers and suppliers such as Voith GmbH, Xerium Technologies, Inc. and its subsidiary Stowe have developed nip presses which utilize sensors imbedded within the sensing roll cover. These nip press generally utilize a plurality of sensors connected in a single spiral wound around the roll cover in a single revolution to form a helical pattern. An individual sensor is designed to extend into the nip region of the nip press as the sensing roll rotates. In this fashion, the helical pattern of sensors provides a different pressure signal along the cross-directional region of the nip press to provide the operator with valuable information regarding the pressure distribution across the nip region, and hence, the pressure that is being applied to the moving web as it passes through the nip region.
Control instrumentation associated with the nip press can provide a good representation of the cross-directional nip pressure (commonly referred to as the “nip pressure profile” or just “nip profile”) and will allow the operator to correct the nip pressure distribution should it arise. The control instruments usually provide a real time graphical display of the nip pressure profile on a computer screen or monitor. The nip profile is a compilation of pressure data that is being received from the sensors located on the sensing roll. It usually graphically shows the pressure signal in terms of the cross-directional position on the sensing roll. The y-axis usually designates pressure in pounds per linear inch while the x-axis designates the cross-directional position on the roll.
While a single line of sensors on the sensing roll may provide a fairly good representation of nip pressure cross-directional variability, these same sensors may not properly take into account the variability of pressure across the nip region caused by the high speed rotation of the sensing roll. The dynamics of a cylinder/roll rotating at a high angular speed (high RPMs) can cause slight changes to the pressure produced by the cylinder/roll that are not necessarily detectable when the cylinder/roll is at rest or rotating at a low speed. Such dynamic changes could be the result of centrifugal forces acting on the cylinder/roll, roll flexing, roll balance, eccentric shaft mounting or out-or round rolls and could possibly be influenced by environmental factors. The dynamic behavior of a typical high speed rotating cylinder/roll is often characterized by a development of an unbalance and bending stiffness variation. Such variations along the cylinder/roll are often referred to as rotational variability. Unbalance can be observed as a vibration component at certain rotating frequencies and also can cause unwanted bending of the flexible cylinder/roll as a function of the rotating speed. Since the lengths of the sensing rolls used in paper manufacturing can be quite long, unbalance in the rotating rolls can pose a serious problem to the paper manufacturer since a less than even nip pressure profile may be created and displayed by the control equipment. Any unwanted bending of the sensing roll can, of course, change the amount of pressure being exerted on the web as it travels through the nip roller. Again, since even nip pressure is highly desired during paper manufacturing, it would be highly beneficial to correctly display the nip pressure profile since any corrects to be made to the rotating roll based on an inaccurate nip pressure profile could certainly exacerbate the problem. A single sensor located at an individual cross-directional position on the sensing roll may not be able to compensate for the effect of rotational variability at that sensor's position and may provide less than accurate pressure readings. There are three primary measurements of variability. The true nip pressure profile has variability that can be term cross-directional variability as it is the variability of average pressure per cross-direction position across the nip. Each sensor in a single line of sensors may have some variability associated with it that may be calculated as the data is collected at high speed. This particular variability profile represents the variability of the high speed measurements at each position in the single line of sensors. This variability contains the variability of other equipment in the paper making process including the rotational variability of the roll nipped to the sensing roll. The third variability profile is the nip profile variability of multiple sensors at each cross-directional position of the roll. This variability represents the “rotational variability” of the sensing roll as it rotates through its plurality or sensing positions.
One of the problems of rotational variability is the creation of “high spots” and “low stops” at various locations along the sensing roll. A single sensor located at a cross-directional position where a high spot or low spot is found could provide the processing equipment with an inaccurate pressure reading being developed at that location. This is due to the fact that the overall pressure that is developed at the sensor's location as the roll fully rotates through a complete revolution will be lower that the measured “high spot” reading. Accordingly, a nip pressure profile which is based on the reading of a sensor located at a high or low spot will not be indicative of the average pressure being developed that that location. The processing equipment, in relying on this single, inaccurate reading, will calculate and display a nip pressure profile which is at least partially inaccurate. If a number of single sensors are located at numerous high or low spots, then the processing equipment will display a nip pressure profile which has numerous inaccuracies. The operator of the papermaking machinery may not even be aware that the processing system is displaying an inaccurate nip pressure profile. Further, attempts to correct the sensing roll based on an inaccurate nip pressure profile could lead to even greater inaccuracies.
Therefore, it would be beneficial if the manufacturer could detect and measure any rotational variability along the length of the covered roll of a nip press and compensate for it when a real time nip pressure profile is being calculated and displayed. The present invention provides a better measurement of the true nip pressure profile and is also capable of providing a previously unmeasured nip profile variability of the rotation (rotational variability). Furthermore, certain arrangements of sensing elements will provide information on the wear of the cover. Compensation for any rotational variability should produce a nip pressure profile which is a more accurate representation of the pressure being developed along the nip region of the press. The present inventions satisfy these and other needs.